After Sept. 11, long lines and detailed security checks at airports marked Americans most direct experience with the "new normal." But in the aftermath of the Madrid train bombings in March, US law enforcement once again is broadening its presence at one of the most basic levels of American life - the nation's railways and subways.

This week Massachusetts transportation officials announced they would begin to randomly inspect passengers' bags and packages on subway and commuter trains.

The permanent inspection policy is the first in the nation. Officials say it will be in place by late July, in time for the Democratic Convention, when highway closures will lead thousands more commuters to the city's rail system.

The move comes as the federal government has begun to test passenger and baggage-screening equipment similar to those used in airports at some train stations.

The desire for a higher standard of security on city trains is reasonable, say experts, and the new programs may help bring commuters some peace of mind.

But many experts question whether a real improvement in security can be made without disrupting train travel. Others point to the difficulty of implementing random searches without unfairly targeting certain ethnic groups, undermining Americans' civil liberties.

"You want to ask how much security you're getting for the imposition you're causing," says James Carafano, a senior fellow at the Heritage Foundation in Washington. "Something like this comes very, very close to having a chilling effect."

More than a month before the policy takes effect, some Bostonians are expressing exasperation that it will complicate their commute. Already, the infamous Big Dig highway project has caused traffic problems for more than a decade. Closure of a major interstate highway during the convention is likely to only exacerbate the tie-ups.

Still, many people here seem willing to put up with transportation officials searching their briefcases and box lunches if it will reduce the risk of terrorism. "Whatever makes us all safe wouldn't bother me," says Kim Andrews, a federal grant manager, while waiting for a subway car in the city's Park Street station.

The federal government began studying rail security shortly after Sept. 11. But it didn't begin experimenting with new technologies in earnest until after the March 11 train bombings in Spain.

Testing security tools

The Department of Homeland Security has been praised by some security analysts for launching pilot programs, rather than implementing untested or overly aggressive policy. Among the new tools tested at stations in Maryland and Washington, D.C.: a walk-through machine that looks for traces of explosives, an "X-Ray" device that examines baggage, and teams of canines that sniff randomly chosen passengers in subway stations.

The government says these tools, including the use of dogs, could be put in place at several important rail hubs in the event of another terrorist incident. "If certain events come up, we could use these technologies in targeted measures," says Darrin Kayser of the Transportation Security Administration.

Security effectiveness questioned

Many experts view the screening technology as the most effective way to enhance security at commuter and metro trains. But even advanced technology has failed to significantly improve security at airports. A report on aviation security by the General Accounting Office found that airline security hasn't improved much since pre-Sept. 11. Others point to the difficulty of screening people in an environment where speed and volume are priorities.

"The railway systems aren't designed for searches, they're designed to get people through quickly," says Kenneth Button, a public-policy professor at George Mason University in Fairfax, Va.

In Boston, a random search of one in 10 people would not significantly boost security, several experts say. Police would have to search a much higher percentage of passengers to deter or apprehend a potential terrorist. And critics worry that it could lead to racial profiling.

If someone is truly determined to plant a bomb, she adds, knowing that the police are only conducting random searches won't act as a strong deterrent.

Fourth Amendment rights

Experts say that, according to legal precedent, subway passengers asked to show police their bags have the right to refuse. If they consent, any contraband found in the bag becomes admissible evidence, whether it be explosives or drugs.

"Although the government has a right to come up and ask for identification and even search your bags, you have the right to say 'no,'" says Tracey Maclin, a law professor at Boston University. "That needs to be made clear.